Musculoskeletal disorders can be characterized as “work-related diseases.” The World Health Organization (WHO) has described these diseases as multifactorial, with work contributing significantly, although not exclusively, to their causation. The term “disorder” is more appropriate when some of the outcomes of these conditions are of uncertain pathogenesis and may consist of symptoms without obvious clinical signs. The term “work-related musculoskeletal disorders” (WMSDs) has come to replace “repetitive strain injuries” or “cumulative trauma disorders.” The National Safety Council Standards Committee, accredited by the American National Standards Institute, has combined the etiologic notions implied in these terms to define a musculoskeletal disorder (MSD) as a disturbance in the regular or normal function of muscle, tendon, tendon sheath, nerve, bursa, blood vessel, bone, joint, or ligament that results in altered structure or impaired motor or sensory function. Accordingly, WMSDs are MSDs that may be caused, aggravated, or precipitated by intense, repeated, or sustained work activities with insufficient recovery. WMSDs generally develop over a period of weeks, months, or years. It follows that MSDs can be partially caused by adverse work conditions, can be exacerbated by workplace exposure, and can impair work capacity. The National Research Council adopted a similar definition but emphasized that none of the common MSDs are uniquely caused by work exposure and that physical and social aspects of life outside work need to be considered. In this respect MSDs would be more accurately defined as activity-related conditions rather than work-related ones. Nevertheless, this chapter will focus on the impact of occupational factors.
MSDs of the upper extremities are diffuse neuromuscular illnesses with significant proximal upper body findings that affect distal function. The boundaries of the shoulder region are not clearly defined because the neck, shoulder, and upper part of the arm operate as a functional unit. A further complication is that most of the musculoskeletal problems of this region are nonspecific and without well-defined diagnoses. Apart from variable location, the current definitions of MSDs lack criteria for the intensity, frequency, and duration of the symptoms that would be indicative of a “case.” Case status is currently based on symptoms that have occurred within a specific time frame (such as 1 week), at a specific frequency (such as three episodes in the past year), for a given duration (such as a single episode lasting more than 5 days), or for a combination of frequency and severity.
In this chapter occupational shoulder disorders (OSDs) are defined as work- or activity-related MSDs of the shoulder region. Diagnoses include trapezius and parascapular myalgia, rotator cuff and bicipital tendinitis, impingement syndrome, and subacromial/subdeltoid bursitis. Tension neck and cervical syndrome may be considered as being alternative terms for trapezius myalgia. This chapter will cover primarily the myalgias, as the other conditions have been covered elsewhere in this textbook.
The chapter excludes several other clinical diagnoses. Primary frozen shoulder is idiopathic, and secondary frozen shoulder is usually due to the progression of one of the aforementioned conditions or is a consequence of systemic disease. Frozen shoulder therefore does not conform to the definition of an MSD. Thoracic outlet syndrome (TOS) has been defined as an essentially vascular phenomenon that can be objectively documented. Neurogenic TOS however is a controversial diagnosis that may often rely on physical findings when laboratory tests are often negative. Pascarelli and Hsu encountered neurogenic TOS in 70% of shoulder and upper extremity patients, mostly computer operators and musicians, and postulated that the condition is related to postural derangement. Although thoracic outlet decompression has been advocated in the past to treat a presumed compressive etiology, recent literature has suggested that workers with a diagnosis of TOS who underwent surgery were more likely to be disabled at 1 year and were 21% more likely to describe new neurologic findings when compared with workers that did not undergo surgery. Some state workers’ compensation boards now require objective evidence of brachial plexus involvement prior to authorizing decompression for TOS; this has resulted in denial of the vast majority of procedures proposed in recent years. We cannot comment therefore on the etiologic relationship of TOS to work-related disability, and we do not consider this condition to be an OSD at this time.
The chapter reviews the epidemiology, etiology, and suggested treatment of OSDs. It also explores the relationship of acromioclavicular and glenohumeral arthritis to occupational factors and provides general information on the primary and secondary prevention of MSDs in occupational settings. The workers’ compensation system has been implicated as a contributing factor in the reporting of MSDs. Therefore the influence of workers’ compensation on the outcome of treatment is examined. Finally, the chapter reviews the current disability compensation systems.
Occupational Shoulder Disorders
This chapter uses the descriptive term occupational shoulder disorder (OSD) to denote WMSDs of the shoulder. The alternative term occupational cervicobrachial disorders appeared mainly in the Japanese, Australian, and Scandinavian literature in the 1970s and 1980s but has not been adopted in the United States. Another term work-related upper extremity musculoskeletal disorders has also been used, but it includes the elbow, forearm, wrist, and hand. Similarly, the Cochran Collaboration review coined the term complaints of the arms, neck, and shoulders (CANS), defined as “musculoskeletal complaints of arm, neck and/or shoulder not caused by acute trauma or by any systemic disease.” The factor “Work-relatedness” is not mentioned in the CANS model. Ergonomic workloads, such as repetitive and forceful motion, work organizational factors, and psychosocial work factors have definitely been implicated as causes of CANS but these risk factors are activity-related rather than work-related. A well-known clinical problem concerning musculoskeletal disorders of the shoulder, such as tendonitis and bursitis, is that they are difficult to differentiate but can be identified as a group. Consensus was therefore reached to use the term subacromial impingement syndrome in CANS for a disorder that includes the rotator cuff syndrome; tendonitis of the infraspinatus, supraspinatus and subscapularis muscles; and bursitis in the shoulder area. Specific complaints included under CANS are cervical disk hernia, frozen shoulder, and subacromial impingement syndrome (tendinitis and bursitis of the shoulder and rotator cuff syndrome).
The term OSD refers to a symptom complex that is characterized by vague pain about the shoulder girdle, including the paracervical, parascapular, and glenohumeral musculature. It may also be associated with pain that radiates into the region of the upper part of the arm. OSDs are thought to be the result of cumulative trauma associated with the performance of certain activities and tasks.
The variable definitions of the disorders make it difficult to estimate the burden of OSDs in the general population. The National Health Interview Survey for 1995 showed a 1.74% prevalence of impairment from upper extremity or shoulder MSDs. However, estimates of incidence in the general population, as opposed to the working population, are unreliable because more than 80% of the adult population in the United States is in the workforce.
The annual survey of occupational injuries and illnesses conducted by the Bureau of Labor Statistics (BLS) is the most frequently referenced source of information on WMSDs in the United States. The U.S. Department of Labor defines an MSD as an injury or disorder of the muscles, nerves, tendons, joints, cartilage, or spinal discs. MSDs do not include disorders caused by slips, trips, falls, motor vehicle accidents, or similar accidents. In 2013 there were 307,640 cases of MSDs, accounting for 33% of the injuries and illnesses that involved days away from work—similar to previous years. Shoulder injuries amounted to 86,690 or 7.5% of all nonfatal injuries and 23% of all WMSD. The incidence of shoulder injuries was 8.2 per 10,000 full-time employees, with the highest incidence (12.9 per 10,000) in local government. The median time away from work due to an occupational shoulder injury was 24 days; this was longer than the median time for all MSDs combined. Sprains and strains accounted for 63% of the shoulder injuries, followed by soreness (22%). The incidence rate for males was higher than that for females (9.1 vs. 7.0 per 10,000 full-time workers), with the highest rate (10.4 per 10,000) found among those aged 45 to 54 years.
Courtney and Webster cross-tabulated the BLS data and found that the most frequent shoulder injuries were sprains, strains, and tears caused by overexertion; these ranked second to overexertion back injuries. In half the cases these injuries resulted in 6 days’ absence from work. The most severe shoulder injuries however were those with “general symptoms” resulting from bodily reaction and overexertion, with a median of 128 days away from work.
The Safety & Health Assessment & Research for Prevention program of the Washington State Department of Labor and Industries used Washington State Fund-accepted claims data to estimate the burden of workers’ compensation claims for rotator cuff syndrome [International Classification of Diseases (ICD-9-CM) codes 726.1, 726.11, and 840.4; Current Procedural Terminology codes 23410, 23412, 23415, and 23420] and shoulder WMSDs. Men accounted for about two thirds of the claims, and the median age of the claimants was in the mid-30s. Of all WMSDs of the upper extremity, rotator cuff syndrome incurred the highest median cost per compensable claim: the cost increased from $3,570 in 1987 to $9,410 in 1992 and then decreased to $6,462 in 1995. The median cost of shoulder WMSDs in that period was $350. For both rotator cuff syndrome and shoulder WMSDs, the mean burden was much higher than the median because of skewed distributions; mean costs were $15,790 per case of rotator cuff syndrome and $7,980 per shoulder WMSD. The mean time lost per claim was 263 days (median, 97) for rotator cuff syndrome and 213 (median, 41) for shoulder WMSDs. The data for Washington State are likely to be representative of other states, as the data for WMSDs of the upper extremity collected by Liberty Mutual across the United States (covering 10% of the private workers’ compensation market) were similarly skewed.
Most of what is known about the etiology of MSDs comes from epidemiologic studies. However, the published prevalence and incidence data should be viewed critically. As discussed earlier, there is a lack of agreement on case definitions, which has resulted in epidemiologic studies reporting widely different estimates for the burden associated with these disorders. In the literature reporting on neck and shoulder WMSDs, results from contrasting definitions have produced differences in prevalence (55% vs. 20% to take the most extreme), overall disability (14.6% vs. 23.2%), difficulty at work (8% vs. 15.5%), and the proportion of patients reporting that pain interfered with work (27.3% vs. 16.2%). Studies using different case definitions therefore lack comparability. Furthermore, this literature demonstrates that MSDs are not unique to any specific occupational group, with reported occupations ranging from meat processors to apparel workers or assemblers in the manufacturing industries to office-based data entry operators. Etiologically therefore MSDs may be activity-related rather than work-related, and further discussion will focus on the roles of type, intensity, and duration of activity as they relate to OSD.
The common trait of the occupational groups for which high OSD rates are reported is intense exposure to specific activities or work attributes. The attributes associated with an increased probability of MSDs are considered to be risk factors. It is plausible that individuals who engage in light static occupations would be more likely to suffer from trapezius myalgia, whereas those involved in heavy labor would be more susceptible to rotator cuff disease. However, this hypothesis still needs to be fully verified. Nonetheless, attempts have been made to improve understanding of the relationships between activity and OSD. In 1997 the National Institute of Occupational Safety and Health (NIOSH) studied the epidemiologic evidence for the work-relatedness of neck, shoulder, and upper extremity disorders. The focus of the review was to assess the evidence for relationships between MSDs and workplace exposure to the following factors: (1) repetitive exertion, (2) awkward posture, (3) forceful exertion, and (4) hand-arm vibration. The review included studies relevant to shoulder disorders defined by a combination of symptoms and physical examination findings or by symptoms alone, but not specifically defined as tendinitis. NIOSH also included studies for which the health outcome combined neck and shoulder disorders (tension neck, cervical syndrome, TOS, frozen shoulder, tendinitis, and acromioclavicular syndrome), but in which the exposure was likely to have been specific to the shoulder. Diagnoses of shoulder disorders were based on symptoms determined by interview and physical examination. Shoulder tendinitis included supraspinatus, infraspinatus, and bicipital tendinitis.
The review used five criteria for assessing the evidence: strength of association, temporal relationship, consistency of association, coherence of evidence, and the exposure-response relationship. The included studies generally compared workers in jobs that involved higher levels of exposure with workers who had lower levels of exposure, with the degree of exposure determined by observation or measurement of job characteristics. The resulting epidemiologic evidence for the risk factors of upper extremity MSDs is summarized in Table 21-1 . In the following we focus on risk factors that could be associated with OSDs.
|MSD Location or Diagnosis||No. of Studies||Force||Static or Extreme Postures||Repetition||Vibration (Segmental)||Combination|
|Neck and neck/shoulder||>40||++||+++||++||+/0||−|
The review found evidence of a positive association between highly repetitive work (factor 1 in the above list) and OSDs. Of the seven studies that met at least one of the specified criteria, five reported significant associations (with odds ratios [OR] ranging from 1.6 to 5.0). However, the evidence has important limitations. Only three studies specifically addressed the health outcome of shoulder tendinitis, and these studies involved exposure to repetitive activities combined with awkward shoulder postures or static shoulder loads. The other six studies with significant positive associations dealt primarily with symptoms.
The NIOSH review also found evidence of a relationship between OSDs and repeated or sustained shoulder postures with greater than 60 degrees of flexion or abduction (factor 2). Of 13 studies examined, seven reported significant risk estimates (OR, 2.3 to 10.6). There was evidence for both shoulder tendinitis and nonspecific shoulder pain. The evidence for risk involved in maintaining specific shoulder postures was strongest in those with combined exposure to several physical risk factors, such as holding a tool while working overhead. The association was positive and consistent in the six studies that used diagnosed cases of shoulder tendinitis or a constellation of symptoms and physical findings compatible with tendinitis as the health outcome. Only one of the 13 studies failed to find a positive association between exposure and symptoms or a specific shoulder disorder. This result was consistent with evidence in the biomechanical, physiologic, and psychosocial literature.
The NIOSH 1997 report found insufficient evidence for a positive association between shoulder MSDs and either force (factor 3) or exposure to segmental vibration (factor 4).
When assessing combinations of the aforementioned factors, the NIOSH review found the strongest associations with WMSDs of the lower part of the back. For the upper extremities, NIOSH needed to combine risk factors (such as force exertion and exposure to hand-arm vibration while operating powered hand tools) to estimate some risk indicators for pathology. Few independent studies at that time had attempted to investigate combined effects on OSDs. The combined effect of forceful movement and vibrating machinery was later empirically demonstrated by Armstrong and colleagues, but a true appreciation of the effect of multiple factors remains poorly investigated.
An additional confounding variable on the risk of a given job-related activity may be the environment in which the activity is performed. Hildebrandt and colleagues reviewed 27 studies that related climatic (i.e., damp, wind, or cold) or seasonal (i.e., summer, winter) factors to MSDs, although none of these studies specifically addressed the subject. Additionally, they distributed a questionnaire to 2,030 workers in 24 different occupations and found that a third of the workers-related symptoms at the lower back and neck-shoulder region could be attributed to climatic conditions, perceiving that these conditions either caused or aggravated their symptoms. Sick leave as a result of neck-shoulder symptoms was associated with climatic factors, particularly draughts and wind. The authors concluded that researchers, workers, and patients consider such a relationship plausible; however, the epidemiologic evidence is still weak.
OSDs are multifactorial in origin and may be associated with both occupational and nonoccupational factors. The relative contributions of these covariates may be specific to particular disorders. For example, the confounders for nonspecific shoulder pain may differ from those for shoulder tendinitis. Two of the most important confounders or effect modifiers for shoulder tendinitis appear to be age and sport activities. Subjects who have been extremely active in sports seem to have an increased risk for shoulder tendinitis and acromioclavicular osteoarthrosis, with those who have been extremely active in sports and also report high exposure to load lifting during work at even greater risk. In other words, sports activities add to the workload on the shoulder and increase the risk for OSDs. Most of the shoulder studies considered the effects of age in their analysis. However, the NIOSH review concluded that it is unlikely that the majority of the positive associations between physical exposure and OSDs are due to the effects of non-work-related confounders, of which age represents the most significant confounder.
The etiology of OSDs, in particular trapezius myalgia, is uncertain. Two theories have been proposed: the organic or physiologic theory and the psychosocial theory .
The organic or physiologic theory is based on the premise that shoulder pain is secondary to statically sustained contraction of the trapezius. Sustained contractions result in increased intramuscular pressure and decreased blood flow. Ischemic conditions occur when intramuscular pressure exceeds the capillary closing pressure at about 30 mm Hg. The increased metabolic demands of the working muscle and the relative ischemia caused by increased intramuscular pressure may contribute to derangements in the balance of intracellular pH/lactic acid, calcium, and potassium.
In a study of 20 assembly line workers with neck and shoulder pain, Bjelle and colleagues found significantly high levels of muscle enzymes, including creatinine phosphokinase and aldolase, in eight workers without any underlying pathology. The elevated muscle enzyme levels were found to diminish after 2 to 8 weeks of sick leave. In addition, elevated serum creatinine kinase levels have been observed in welders, cash register operators, and assembly line workers but not in control groups consisting of controllers and forklift drivers. The sustained load necessary for light, static work has been theorized to cause severe adenosine triphosphate depletion, increased permeability, and the resultant release of muscle enzymes.
The level of activity at which a static, isometric contraction of shoulder muscles causes injury is unknown. Several researchers have attempted to identify an endurance limit, defined as the highest force that can be maintain for an “unlimited” period. Jonsson and colleagues suggested that the static load level should always be below 5% of the maximal voluntary contraction (MVC). In support of this suggestion Sjogaard and colleagues have shown that muscle fatigue occurs at 5% of MVC after 1 hour of sustained contraction. This endurance was subsequently confirmed by Frey Law and Avin specifically for the shoulder muscles; these authors also provided data to suggest that shoulder muscles were more fatigable than the trunk muscles, which could endure this exertion level for approximately 5 hours. Furthermore, multiple electromyography (EMG) studies have suggested that myalgia patients have abnormally elevated muscle tension. Findings among these patients include (1) higher muscle tension in symptomatic patients, (2) higher muscle tension during sleep, (3) higher muscle tension at the painful site, (4) pain even when static muscle contraction is as low as 2% to 5% of MVC, (5) faster fatigue on the painful side, and (6) shorter muscle endurance. EMG findings that showed decrements in the generation of muscle force during repetitive use have provided evidence of either transient motor unit fatigue or permanent skeletal fiber damage.
One theory for decreased fatigue resistance and force generation in myalgia patients is the “Cinderella hypothesis.” This hypothesis suggests that muscle damage may be mediated by mechanisms related to muscle recruitment. According to this theory, relatively small, low-threshold, type I motor units are persistently activated and loaded. These are the first motor units recruited for low-force, repetitive endurance work. They remain in action throughout low-level contractions. Because they carry a disproportionate burden, they are referred to as “Cinderella” fibers. Sustained contraction and activation of these motor units causes pain and fatigue and may eventually lead to permanent injury. Muscle fiber damage may ultimately result in interstitial myofibrositis with a persistent reduction in blood flow. Larsson and colleagues performed bilateral open biopsies of the trapezius muscle in 17 patients with chronic myalgia related to static loads during repetitive assembly work and concluded that there was an association between clinical myalgia and the presence of pathologic muscle fibers on histology. Additional muscle biopsy studies have demonstrated degenerated mitochondria and increased glycogen deposits. Fiber structural damage is also accompanied by products of cell inflammation and necrosis, edema, and leakage of intrafiber proteins and enzymes.
With dropout of some of the muscle fibers, the overall resistance of the motor unit to fatigue is reduced. Clinically, localized fatigue, muscle strain, and pain can occur with very low level contractions, such as those needed to hold the arms in an elevated posture. The trapezius muscle has been found to be affected by jobs that require static muscle overload. For example, a high prevalence of OSDs has been reported among dental care workers; some were 5.4 times more likely to experience symptoms than a control group of pharmacists. This increased risk is considered to be secondary to maintaining an unsupported awkward working posture with cervical flexion of 45 to 90 degrees and shoulder flexion and abduction of more than 30 degrees for extended periods ( Fig. 21-1 ). Excessive scapular elevation as a result of mental stress or workstation design may also contribute to the increased trapezius load.
In contrast to the organic/physiologic theory, the psychosocial theory maintains that emotional stress is an etiologic factor in the development of OSDs. The proponents of this theory contend that OSDs occur in jobs that do not involve excessive muscle strain and consequently are not related to overuse but are more a result of psychosocial factors. In a study of 607 metal industry workers, depression and distress symptoms were found to be predictors of low back pain, neck-shoulder pain, and other musculoskeletal complaints. Workers may fear that if they ignore their symptoms, the symptoms will progress and become permanently disabling. The workers’ compensation system can contribute to this problem by awarding benefits based on the recognition that cumulative trauma can cause significant disability. In a study of 201 patients with chronic pain, Tait and colleagues found that patients with litigation claims reported pain of significantly longer duration and had significantly greater disability than nonlitigating patients. Bongers and colleagues found that monotonous work, a high-perceived workload, and time pressures were causally related to musculoskeletal symptoms.
In an attempt to address the complex, multifactorial nature of OSDs, Armstrong and colleagues proposed a model that incorporates both the organic and the psychosocial theories. In this model exposure refers to external factors, such as work requirements. The external exposure produces an internal dose , which in turn disturbs the internal state of the individual. Such disturbances may be mechanical, physiologic, or psychologic and in turn evoke a certain response , mechanical and/or metabolic changes, that occurs at the tissue level. Finally, capacity , which can be either physical or psychologic, refers to the ability of the individual to resist destabilization after various doses of exposure. This model provides a framework to explain the relationship between work exposure factors and the different responses that occur, both psychologic and physiologic. A similar model was adopted by the panel of the National Research Council.
Evaluation of a patient with a suspected OSD involves a thorough history and physical examination. The hallmark of OSDs is musculoskeletal pain or discomfort that occurs on the job. Symptoms will vary based on the patient’s specific diagnosis. Usually, the underlying diagnosis will fall into one of the following categories: myalgia, tendinitis, or bursitis. In a prospective study of 204 workers with occupationally related upper limb or neck pain, Sikorski and colleagues found that a discrete MSD existed in 58% of cases.
A large proportion of patients diagnosed with an OSD will have signs and symptoms consistent with trapezius myalgia. Symptoms of trapezius myalgia typically include muscle fatigue and stiffness accompanied by subjective pain or headaches. On examination, patients may demonstrate muscle tightness, increased tone, and multiple “trigger points.” Occasionally, subtle decreases in range of motion will be seen. Risk factors for myalgia are unvarying stationary positioning of the shoulder and neck, along with prolonged static loading.
Neurologic and vascular conditions such as cervical radiculopathy, TOS, and Raynaud phenomenon could also be associated with OSDs. Congenital or developmental deformities of the shoulder or cervical spine can predispose a worker to OSDs and may need to be addressed. Musculoskeletal neoplasms, both benign (such as osteochondroma) and malignant, can be found about the shoulder girdle and should be considered. Referred sources of pain from other organ systems, including the cardiac, pulmonary, and gastrointestinal systems, should be ruled out.
Several classifications of OSDs have been proposed, and such systems have been used for the development of treatment protocols. A five-grade classification system was developed by the OCD committee of the Japanese Association of Industrial Health ( Box 21-1 ). This system includes tendinitis as well as several neurologic and vascular symptoms that often accompany occupational shoulder pain. A simplified three-stage system was developed by the Occupational Repetition Strain Advisory Committee in Australia. This system is based on the persistence of the symptoms and interference with work ( Box 21-2 ).
Subjective complaints without clinical findings
Subjective complaints with induration and tenderness of the neck, shoulder, and arm muscles
Includes grade II and any of the following:
Increased tenderness or enlargement of affected muscles
Positive neurologic tests
Decrease in muscle strength
Tenderness of spinous processes of the vertebrae
Tenderness of the paravertebral muscles
Tenderness of the nerve plexus
Tremor of the hand or eyelid
Kinesalgia of the neck, shoulder, and upper extremity
Functional disturbance of the peripheral circulation
Severe pain or subjective complaints of the neck, shoulder, or upper extremity
Severe type of grade III
Direct development from grade II without passing through grade III, but having specific findings as follows:
Orthopedic diagnosis of the neck-shoulder-arm syndrome
Organic disturbances such as tendinitis or tenosynovitis
Autonomic nervous disturbances such as Raynaud phenomenon, passive hyperemia, or disequilibrium
Mental disturbances, such as anxiety, sleeplessness, thinking dysfunction, hysteria, or depression
Disturbances not only at work but also in daily life
Aching and tiredness of the affected limb that occurs during the work shift but subsides overnight and during days off work. There is no significant reduction in work performance, and there are no physical signs. This condition can persist for months and is reversible.
Symptoms fail to settle overnight, cause a sleep disturbance, and are associated with a reduced capacity for repetitive work. Physical signs may be present. The condition usually persists for months.
Symptoms persist at rest. Sleep is disturbed, and pain occurs with nonrepetitive movement. The person is unable to perform light duties and has difficulty with nonoccupational tasks. Physical signs are present. The condition may persist for months to years.
Luck and Andersson proposed a pathophysiologic grading system that is a modification of the Australian classification ( Box 21-3 ). This focuses on myogenic pain. The pathophysiologic basis for pain in grade I is metabolic changes that occur in response to a sustained static load. Progression to grade II involves pain that does not resolve overnight and is secondary to muscle inflammation and early interstitial fibrosis. Grade III is characterized by progression to severe myopathy with interstitial fibrosis. It has been proposed that the more advanced the grade, the longer and more aggressive the treatment must be in terms of both time off work and involvement of a multidisciplinary team.
Grade I (Mild)
Shoulder girdle muscle pain that occurs during work or similar activities and resolves a few hours later; no findings on physical examination.
Grade II (Moderate)
Shoulder girdle muscle pain that persists for several days after work; muscle belly and insertional tenderness on examination.
Grade III (Severe)
Shoulder girdle muscle pain that is constant for weeks or longer; multiple tender areas; palpable induration indicative of muscle fibrosis; muscle belly contracture; reduced range of motion of myogenic origin.
Successful management of the patient requires a multidisciplinary approach. Treatment of physical disorders, psychosocial evaluation, and an appropriate understanding of the work demands are all equally important. Feuerstein and Hickey suggested the use of a multidisciplinary approach that focuses on physical, ergonomic, and psychologic factors. They noted that patients treated with a multidisciplinary approach had a significantly higher rate of return to work than did those treated with usual care. Johansson and colleagues reported a significant decrease in sick leave, pain intensity, and analgesic use in patients treated with a cognitive-behavioral pain management program. The treatment team included a clinical psychologist, physical therapist, occupational therapist, physical education teacher, vocational counselor, physician, and nurse.
The medical/surgical management of rotator cuff disease, biceps tendonitis, bursitis, and adhesive capsulitis has been covered elsewhere in this textbook. The management of myalgia consists of modalities, such as the application of ice and heat. Range of motion exercises together with strengthening exercises should also be instituted. Antiinflammatory agents may be used in moderate cases, supplemented with low doses of tricyclic antidepressants in more refractory cases. A recent evidence-based systematic review provided evidence for the efficacy of conservative treatments in the management of rotator cuff tendonitis, biceps tendonitis, and trapezius myalgia.
Changes in the workplace can also be beneficial. Introducing more frequent rest breaks and altering the posture and exertion of force at work can help prevent or alleviate OSDs. For example, Hallman and colleagues reported that workers who sat for approximately 9 hours a shift were almost 3 times more likely to report high neck and shoulder pain intensity than those who sat for more moderate durations (about 7 hours). Mekhora and colleagues showed that simple ergonomic interventions, such as adjusting monitor height, seat height, and keyboard height could help reduce symptoms in patients suffering from trapezius myalgia. This, in turn, resulted in less lost work time and higher productivity.
Finally, the psychosocial element of OSDs should not be underestimated. High job demands predict future neck/shoulder pain. It has been suggested that high job demands may increase strain and subsequently increase muscle tension or other physiologic reactions that put individuals at a greater risk for developing neck/shoulder pain. Most theoretical models that describe the relationship between occupational factors and musculoskeletal problems, such as the dose-response or biopsychosocial models, assume that psychosocial stressors at work lead to MSD by eliciting physiologic responses (e.g., by increasing the individual’s muscle tension). Thus the psychologic state resulting from the experienced occupational stressors initiates a bodily response that over time may manifest in the form of health problems. Eijckelhof and colleagues found some support for this view: applying simulated workplace stressors (cognitive/emotional stress, work pace, and precision) resulted in increased neck-shoulder and forearm muscle activity; decreased control was not a significant predictor. These elements at work require work organizational changes. In addition to medical management, more aggressive approaches to improve sense of control over symptoms and functional loss, avoidance of unnecessary surgery, assistance to patients in managing residual pain and stress, and attention to employer-employee conflicts are all important in preventing prolonged work disability secondary to upper extremity disorders.
Degenerative Joint Disease
Although the relationship between cumulative trauma and injury to soft tissues about the shoulder girdle has been delineated, the association of glenohumeral arthritis with occupational factors is less clear. Unlike the hip and knee, the shoulder is not a weight-bearing joint, and thus symptomatic degenerative arthritis of the glenohumeral joint is less common.
A few studies have investigated the association of glenohumeral arthritis with various occupations. Although some of these studies have reported a relationship between certain occupations and osteoarthritis of the shoulder, a direct association has not been found. Kellgren and Lawrence and later Lawrence found that the prevalence of glenohumeral arthritis in men was influenced by occupation. Waldron and Cox studied the skeletons of 367 workers buried in London between 1729 and 1869 and found no significant relationship between occupation and osteoarthritis of the shoulder. Similarly, in a study that included 151 shoulder dissections, Petersson did not find any convincing evidence to support the notion that occupation is a factor in the development of osteoarthritis of the glenohumeral joint.
As with OSDs, sustained loading may be associated with the development of glenohumeral arthritis. Dentists seem to be susceptible as a result of sustained static loads while maintaining the shoulder in a position of flexion and abduction with elevation of the scapula. In a Finnish study that included 40 dentists, Katevuo and colleagues found that 46% had radiographic evidence of osteoarthritis and 44% had bilateral disease. In contrast, only 13% in the control group—82 farmers presumably unexposed to static load—had findings consistent with osteoarthritis.
It has been speculated that pneumatic drilling may predispose workers to degenerative arthritis. To examine the effect of vibration exposure on the shoulder, Bovenzi and colleagues compared 67 foundry workers who used vibratory tools with 46 heavy manual laborers. They found no significant difference between the two groups in the prevalence of radiographic changes in the shoulder. In general, it has been difficult to differentiate degenerative changes caused by vibration from those that can simply be attributed to heavy manual work.
Degeneration of the acromioclavicular joint is more common than glenohumeral arthritis. As with glenohumeral arthritis, there is little evidence for a relationship with specific occupations. In a radiographic study of bricklayers and blasters, Stenlund and colleagues found that OR for degeneration of the acromioclavicular joint increased with the level of lifetime weight handled on the job. When adjusted for age, construction workers had more than a two times greater risk for osteoarthritis of the acromioclavicular joint than their supervisors. The authors found that construction workers who engaged in sports activity as well were more susceptible to acromioclavicular osteoarthritis. Unlike tendinitis, the risk for those with high workloads did not show job-specific trends: even though most of the loading at work seemed to have been on the left side, the risk for the left shoulder was no greater than that for the right shoulder. It is possible though that OR for the left side in these studies may have been underestimated.
Other studies have reported contrary findings. In a cadaver study De Palma found degenerative changes in almost all subjects older than 50 years. Petersson identified degeneration often in 30- to 50-year-old individuals and regularly in those older than 60 years. Because degeneration occurred with equal frequency in men and women and was of the same severity in the right and left shoulders, occupation may not have been a contributing factor. In a retrospective study that included 83 patients who underwent distal clavicular resection for arthritis, Worcester and Green found no relationship with occupation.
Although a few studies have suggested that the risk factors for OSDs may apply to arthritis, the insidious onset of degenerative disorders makes them more difficult to attribute to work. In summary, there is little evidence that glenohumeral and acromioclavicular osteoarthritis are work related.
“Ergonomics” is the study of work—the tasks, the technology, and the environment—in relation to human capabilities. In practice, ergonomics is a process of problem solving. This process requires answers to several questions:
Where is the problem? The jobs or positions targeted for intervention
What is the problem? The specific risk factors for MSD present on the job, their magnitude, and the body parts at risk
Why is there a problem? The possible ergonomic root causes of the risk factors, specifically, design hazards that may exacerbate MSDs, such as the design of the workstation, tools, and products that need to be handled, as well as the way that work is organized or the techniques that are used by the individual
What should be done ? Prioritizing hazard control measures
The first two of these points require a surveillance of job hazards. Prevention of WMSDs requires methods that focus on the assessment of risk factors, characterizing the stresses that act on the worker. The assessment should establish the circumstances under which people are affected and the severity of the problems. The physical stresses associated with OSDs rely on the findings of epidemiologic studies. The epidemiologic evidence suggests that several physical stressors play a role in the etiology, either separately or jointly, although the strength of their association with OSDs varies. Therefore the following Occupational data should be collected:
Excessive or sustained exertion of force
Awkward postures, mainly shoulder flexion/extension or abduction
Repetitive motions of the shoulder and neck *
Contact with vibrating power tools
Low ambient temperature
* Studies usually define repetitive work for the shoulder as activities that involve cyclic flexion, extension, abduction, or rotation of the shoulder joint. The studies operationalize repetitiveness in four ways: (1) the observed frequency of movements past predefined angles of shoulder flexion or abduction; (2) the number of pieces handled per time unit; (3) the duration of short cycle time/repeated tasks within the cycle; and (4) a descriptive characterization of repetitive work or repetitive arm movements.Knowledge of the dimensions of the exposure—magnitude, duration, and repetitiveness (frequency)—is necessary to assess the risk. The choice of method is a tradeoff between time, resources, and the level of detail desired. The most cursory task analysis involves a description of the sequence of functions or actions with the use of terms, such as transportation, operation, inspection, or storage. The activities of the upper extremities often require a more detailed analysis, with methods ranging from indirect measures, such as self-reports in interviews or questionnaires, through observations, to direct instrumented measures, such as electrogoniometry, heart rate monitoring, or EMG. These measurements serve to quantify the exposure to risk factors. Muscular stress can be quantified by EMG signals expressed as percent of maximal voluntary contraction or some other reference value. It is not possible to use the arm/hand without stabilizing the rotator cuff girdle and the glenohumeral joint, and therefore work tasks that demand continuous arm movements generate load patterns with a static load component at the shoulder level. The static load of the shoulder muscles may not be high but, as discussed earlier, the low fatigue tolerance of these muscles may cause pain sooner (within about 1 hour of static contraction).
Exposure to external force can be estimated through biomechanical models. Such models for the shoulder are usually based on anthropometric data describing the length of the body segments (hand, forearm, and upper arm), the weight and center of mass of these segments, and their angular configurations with respect to the trunk. The models then calculate the torques acting around the shoulder. Most biomechanical models of shoulder stress underestimate the actual loads. Inertial forces associated with acceleration and deceleration of the body and work object increase the load, and additional loads may also result from antagonistic muscle forces. In addition to the effect on muscle workload, certain shoulder angles produce pressure on internal and surrounding soft tissues. In fact although the required muscle loads actually decrease as the arm-torso angle exceeds 90 degrees, the pressure on soft tissues continues to increase, an effect that is lost in a simple biomechanical assessment.
In summary, the multifactorial nature of WMSDs implies that several categories of ergonomic hazard induce a variety of stressors. It follows that there are often several possible solutions. Exposure to physical stressors can be reduced by modifying the design of the workstation or the tools, redesigning the work objects, reorganizing the sequence of tasks, or implementing any combination of these solutions. The ideal solution would effectively control all or most of the stressors identified on the job. Dealing with multiple risk factors, numerous root causes, and a variety of possible solutions requires the development of professional and administrative strategies.
The strategies adopted to prevent WMSDs can be classified as primary and secondary. Primary prevention addresses the clinical manifestation of a disease before it occurs. Secondary prevention measures attempt to arrest the development of a disease while it is still in the early symptomatic stage. The first aims at groups of workers, whereas the second focuses on the individual.
By following established practices for controlling exposure to hazardous materials, NIOSH lists four areas of strategy to control and prevent musculoskeletal injuries:
Engineering controls to redesign tools, tasks, and workstations.
Administrative controls, including:
Work practices (job rotation or enrichment, limited overtime, and rest breaks)
Safe work practice training, including body mechanics
Worker placement evaluation (employee selection)
Personal protective equipment, such as gloves, padding, and wrist rests and armrests; NIOSH and the Occupational Safety and Health Administration (OSHA) consider braces to be medical devices rather than personal protective equipment.
Medical management to minimize the impact of the health problems.
The goal of NIOSH intervention strategy is to eliminate, reduce, or control the presence of ergonomic hazards. These interventions can be used in both primary and secondary prevention and will be discussed in more detail under secondary prevention.
For primary prevention, NIOSH has recommended a tiered hierarchy of controls in which engineering changes are viewed as the first preference, administrative changes are a second preference, and personal protective equipment is the last choice.
The increase in reported cases of MSD and the increase in workers’ compensation costs in the United States have prompted some regulatory efforts. Until 1991, attempts to standardize or control exposure to MSD risk factors were limited to specific tasks or situations, such as working with video display terminals or exposure to vibration. After high-profile citations, OSHA issued guidelines in 1993 for managing ergonomics programs. These guidelines were limited to the meatpacking industry, an industry targeted because of a high incidence and severity of MSDs of the upper extremities. OSHA further emphasized that these guidelines were not a standard or regulation. OSHA’s approach focused on ergonomics as a process. The guidelines consisted of the following: a discussion of the importance of management commitment and employee involvement, recommended program elements, and detailed guidance and examples for these elements. NIOSH incorporated most of these program elements in its guidelines for setting up an ergonomic program.
OSHA published its Ergonomics Program Standards in the Federal Register (65: 68282-68870) November 14, 2000, but it was rescinded by Congress and President in February 2001. In 2002 OSHA declared its intention to address ergonomics in the work environment in a multi-tiered plan that was to include a combination of enforcement measures, workplace outreach, research, compliance assistance activities, and industry-targeted voluntary guidelines. To date, OSHA has issued guidelines for meat and poultry processing, foundries, shipyards, nursing homes, and retail grocery stores (see www.osha.gov/SLTC/ergonomics ). The plan is designed to reduce musculoskeletal injuries and focuses on prevention. Rather than deploy specific ergonomic regulations, the enforcement relies still on the General Duty Clause, OSHA Act of 1970, Section 5(a)(1), that states that the employer is to furnish each of his or her employees a place of employment that is free of recognized hazards that are causing or likely to cause death or serious physical harm to employees.
In the absence of federal regulations regarding the prevention of WMSDs, some local initiatives attempted to fill the gap, with varying degrees of success. For example, the states of California and Washington introduced their own ergonomic regulations, although those of Washington were withdrawn in 2003.
Unlike the program standards, the American Conference of Governmental Industrial Hygienists chose to develop a performance standard for hand activity by using an expert consensus process (Physical and Biological Hazards of the Workplace). Although acceptable work design standards for preventing OSD have not yet been established, recommendations may be made regarding the prevention of localized fatigue. While prevention of fatigue is desirable in its own right, it may be a precursor of MSD. Recommended acceptable exposure limits are 10% of MVC for continuous work and 17% for intermittent work. However, as noted earlier, such guidelines do not take into account the specific tolerance of the shoulder muscles.
In order to estimate the biomechanical load on the body, it is important to know the locations of the manual controls and where materials, parts, and tools are stored and used, as well as the forces required to handle the work objects. Whether estimated as forces through biomechanical models or as muscle tension by EMG measurements, the workload needs to be compared with the individual’s strength. Anthropometric tables provide some information on the strength of joints and hands by age and occupation. The analyst may either select a value from the literature that corresponds to the population of interest or use data provided by functional evaluation of a specific individual.
In secondary prevention the control strategies focus on individual patients. Ergonomic accommodations for employees with MSDs fall under the category of secondary prevention. Accommodations are interventions intended to reduce exposure to factors that limit the activities of an individual with impairment. More detailed knowledge is required about the residual abilities and limitations of the individual. Design for an individual requires a functional assessment that specifies that person’s strength limits. The clinical evaluator should work closely with the designer during and after design implementation to ascertain that the job can be performed without risk of injury or reinjury.
Three principal means of accommodating impaired or disabled employees may be implemented: client matching, job restructuring, and job modifications.
Client matching , a form of employee selection, is the simplest and most effective way to return a person with a disability to work. It involves ensuring that the job requirements are consistent with the present abilities of the employee. If an employee cannot return to the previous job, an alternative job is found that can be performed without risk of reinjury. This strategy does not attempt to fit the job to the worker because it requires hardly any modifications to the job.
Job restructuring is an administrative control for reducing exposure to a risk factor. Two techniques have been proposed for reducing static load on the shoulder: the introduction of rest breaks and job rotation.
Many studies have attempted to find the optimal rest break frequency, duration, and content to prevent shoulder disorders caused by static, repetitive work. Many of the studies have focused on work at video display terminals (VDT). NIOSH has recommended a 15-minute rest break after 1 to 2 hours of VDT work. Similarly, the Swedish National Board of Occupational Safety and Health has recommended an upper limit of 1 to 2 hours of continuous video display terminal work. The optimal frequency and length of breaks will depend mainly on the type of work that is being performed, the length of time that it can be sustained, and the posture or load that is held. Breaks can be active, with static load being relieved by dynamic muscle work. Short exercise and stretch periods are active breaks and they seem to be more effective than those that involve complete rest.
Static loading of the shoulders can be avoided by rotating from one job to another. This technique requires careful assessment of task demands to ensure that the shoulders will be relieved occasionally. Although this technique is common in manufacturing industries, it is difficult to introduce in office work, which involves fewer tasks with sufficient variability. Workers may however arrange their tasks so that the tasks will periodically take them away from their workstation; for example, they may interrupt typing activities with photocopying, filing, or other duties. This technique can be viewed as a form of active break rather than formal job rotation.
In secondary prevention, job restructuring entails assigning the impaired employee to restricted duties. For example, restructuring the job by assigning heavy lifting tasks to another person would enable a worker with an OSD or low back pain to work while recovering from the injury.
Job modifications and redesign usually involve the use of assistive technology to enable individuals to perform the required tasks. Other employees may also benefit from similar devices. Occasionally, some tasks can be eliminated during the process of introducing new technology.
The following are examples of two cases involving the secondary prevention of OSDs. The first outlines the medical management of a clerical worker with an OSD, and the second demonstrates the application of the ergonomic solving process in secondary prevention.
A 41-year-old, right-hand-dominant secretary at a large investment banking corporation had a chief complaint of right shoulder pain. She described a gradual onset of pain that began approximately 1 year before evaluation. She reported diffuse, poorly localized pain about the right side of her neck and right shoulder region. Her pain was intermittent and varied in terms of severity. In general however her pain worsened with work and was alleviated with rest. Specific work activities that exacerbated her symptoms included typing on a keyboard and writing. She spent approximately 9 hours a day in front of a VDT. Initially, her pain occurred solely during work and seemed to resolve at night.
Over the few months prior to the evaluation however her pain lingered into the evening and occasionally persisted into the initial part of the weekend. She had been evaluated previously by several physicians and underwent a rheumatologic workup for inflammatory disease, for which the result was negative. She also underwent electrophysiologic studies, again with negative results. Her physical examination was negative other than the finding of diffuse tenderness of the right trapezius muscle belly. The results of radiographs of the cervical spine and shoulder were negative.
An OSD was diagnosed, and she started on a course of physical therapy that included shoulder and neck range of motion and stretching exercises. She was instructed in muscle relaxation techniques and counseled with regard to limiting the number of hours spent at the computer keyboard, as well as to the value of rest breaks during her workday.
Her workstation was evaluated by an ergonomist, and several modifications were implemented. These modifications included the procurement of a chair with height adjustment, a wider computer keyboard, and an adjustable stand for holding hard copy being transcribed.
After 6 months her symptoms were much improved, and by 1 year she experienced only minor discomfort, which occurred exclusively during her workday and responded to basic stretching maneuvers.
A 43-year-old woman complained of pain in her right shoulder, elbow, and index finger. She was living in a suburban area, divorced without children. She had been experiencing the symptoms for about 6 months, since a few weeks after starting working on the assembly line of a plant that produced electric engines for adjusting car seats. At first, the symptoms appeared during the second part of her shift work and usually disappeared on the weekend. When a new engine was introduced, production quotas increased. The symptoms became more severe and frequent, and she sought medical care. OSD and lateral epicondylitis were diagnosed. She was on leave for 1 week but as she was being paid by the piece (i.e., by the number of engines she worked on), she preferred to return to work part time. She experienced difficulty with shopping, dressing, and washing. OSHA had cited the plant for various violations, including underreporting of WMSDs. A review of the recordable cases revealed that the incidence rate of WMSDs at the plant was 13 times higher than the national rate for the manufacturing sector.
As part of medical management of the case, an ergonomic assessment of the workstations on the assembly line was conducted. The patient was observed at her position while she inserted an element into the assembled fixture with a magnetic clip inserter. The results of the assessment are summarized in Box 21-4 . Detailed analysis revealed that the task required six operations with the right hand, one of which entailed an awkward posture, exertion of force, and repetitive motion. While using the clip inserter, the right shoulder had to be elevated; the arm was abducted more than 45 degrees and stabilized to enable the transmission of force to the hand tool through the forearm and wrist. The hand tool could not be grasped in a power grip and needed to be steered with the index finger. The operation also required leaning forward while flexing the neck to see the insertion.